RATCHET AND PAWL
A ratchet is a mechanical
device that allows continuous linear or rotary motion in only one direction
while preventing motion in the opposite direction. Ratchets are widely used in
machinery and tools. Though something of a misnomer, "ratchet".A
ratchet consists of a round gear or linear rack with teeth, and a pivoting,
spring-loaded finger called a pawl that engages the teeth. The teeth are
uniform but asymmetrical, with each tooth having a moderate slope on one edge
and a much steeper slope on the other edge.When the teeth are moving in the
unrestricted (i.e., forward) direction, the pawl easily slides up and over the
gently sloped edges of the teeth, with a spring forcing it (often with an
audible 'click') into the depression between the teeth as it passes the tip of
each tooth. When the teeth move in the opposite (backward) direction, however,
the pawl will catch against the steeply sloped edge of the first tooth it
encounters, thereby locking it against the tooth and preventing any further
motion in that direction.
Backlash
Because the ratchet can only stop
backward motion at discrete points (i.e., at tooth boundaries), a ratchet does
allow a limited amount of backward motion. This backward motion—which is
limited to a maximum distance equal to the spacing between the teeth—is called
backlash. In cases where backlash must be minimized, a smooth, toothless
ratchet with a high friction surface such as rubber is sometimes used. The pawl
bears against the surface at an angle so that any backward motion will cause
the pawl to jam against the surface and thus prevent any further backward
motion. Since the backward travel distance is primarily a function of the
compressibility of the high friction surface, this mechanism can result in
significantly reduced backlash.
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